206 



Instructions for Making and Registering 



[Jan. 



since the last observation, a fresh bucketfull should be taken up and the 

 observation repeated. It should also be noticed whether rain has fallen 

 since the last observation. A sudden degression of 3° or 4° indicates 

 the near approach of land. In a voyage from England, lately made by 

 a member of this committee, the temperature of the surface water fell at 

 once 9° Fahr. on approaching within a few miles of the entrance of 

 Table Bay. 



The temperature of the sea at considerable depths can hardly be re- 

 garded as a subject of ordinary meteorological inquiry and regular regis" 

 try, though undoubtedly one of much physical interest, for which reason 

 it is not considered necessary to dwell further on it. 



Of the Hygrometer, §c. — In the absence of Daniell's Hygrometer, or 

 of ether to cool it, the degree of dryness of the air may be ascertained by 

 observing the temperatures marked by two thermometers suspended 

 freely side by side (but not in contact) in the shade, and completely 

 defended from all radiation to or from the sky, the one having its bulb and 

 stem naked, the other with the bulb and lower part of the stem wrapped 

 in linen or cotton, and thoroughly wetted with pure spring or rain water. 

 The temperatures indicated by both should be noted when the wetted 

 thermometer refuses to sink lower, and the conclusions left for subse- 

 quent calculations. The naked thermometer may be the " External 

 Thermometer" itself, in which case a coated thermometer may be kept 

 always suspended near it, completely screened, as above mentioned, and 

 wetted some minutes previous to the regular daily readings. 



If a hair hygrometer be used, its points of absolute moisture and dry- 

 ness should be frequently ascertained, as they are apt to change. The 

 former may be found by keeping it some time in a close covered jar lined 

 with wet blotting paper, and having water in it, and noting the point of 

 moisture beyond which it refuses to go. The latter, by keeping it in 

 the same manner in ajar perfectly air-tight, over fresh burnt quicklime, 

 till it refuses to indicate a higher degree of dryness. 



The best measure of the momentary evaporating power of the air, 

 seems to be the depression of the wetted thermometer below the dry 

 one. But the actual evaporation from a given suyface, is quite another 

 thing, and a question may very reasonably be raised, how far any useful 

 approximation to a knowledge of the total evaporation from an extensive 

 and diversified surface, unequally moistened, and variously exposed to 

 the sun, defended by clouds, or refreshed by dews, can be obtained by 

 any small or local experiments. 



The Rain-gauge is an instrument of such extremely easy construction 

 that any person who lives near a tin-man can procure one. In a climate 

 so arid as that of Africa, however, it must be remembered that it will 

 often need examination and cleansing, owing to long intervals of disuse 

 -in which insects and dust may lodge. It will often happen, too, that the 



